Answer:
(Fatwa: 2368/H=829/th=1431)
The proof of dua after fardh salah is from the holy Prophet Muhammad (PBUH) verbally and practically which are established by ahadith. Thus, it is not bidah (innovation). Those who say it bidah are ignorant.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband
